The present invention relates to a peripheral device with access control and, in particular, to a programmed digital data processing system including a watchdog timer. The microcontroller is generally equipped with a plurality of peripheral devices. Some of these devices control important functional systems of the digital data processing system. These important functional systems can be, for example, a watchdog timer, an interrupt controller, a coprocessor, etc. These peripheral devices should work properly at all times. Therefore, access to registers which are associated with these peripheral devices are restricted.
Different access control systems are known in the art. For example, U.S. Pat. No. 5,073,853 discloses a data processing system including a microcomputer. This microcomputer is provided with a watchdog arrangement comprising a clock counter and an output which is coupled to a reset input of the microcomputer. In order to increase the number of malfunctions to which the watchdog responds, the microcomputer is arranged to repeatedly generate reset signals. These reset signals are in the form of a complete byte X or Y which alternate. These two bytes are applied to an input of a comparator which compares them with identical bytes fed to a further input. The watchdog arrangement is reset only if the bytes fed to the further input are identical to either byte X or Y. This arrangement prevents the watchdog timer from being reset accidentally. Therefore, U.S. Pat. No. 5,073,853 does not provide a completely secure system.